1. Field of the Invention
The present invention generally relates to an optical amplifier, and, more particularly, it relates to a Raman amplifier.
2. Description of the Related Art
In wavelength division multiplexed (WDM) systems used in current optical communication systems, as methods for increasing transmission capacity, there are a method for expanding a signal wavelength band to increase the number of multiplexed wavelengths, and a method for enhancing a transmission rate (bit rate) per wavelength. In normal WDM systems, due to limitation in gain wavelength band of an Erbium doped fiber amplifier (EDFA), most generally, a wavelength for optical signal is selected from a wavelength band (called as a C-band) of about 1530 to 1565 nm. On the other hand, since optical amplification can be effected by the EDFA also in a wavelength band (called as an L-band) of about 1575 to 1610 nm, recently, WDM systems regarding this band has been developed.
By expanding the wavelength band as mentioned above, capacity which can be transmitted by a single WDM system can be increased. Regarding the WDM systems, since a C-band WDM system has firstly been developed, in order to increase the transmission capacity of the WDM system, it is desirable that the existing equipment for the C-band WDM system is utilized and an L-band WDM system is added to the equipment to gradually increase the capacity. In the conventional WDM systems using the EDFA, the transmission rate per wavelength has been enhanced (speeded up) by technically improving various elements constituting a transmission system. However, in systems utilizing a discrete amplifier such as EDFA, achievement of higher speed has been approaching to its limit. In order to achieve further high speed and/or longer distance transmission, it is said that incorporation of a distributed amplifier such as a Raman amplifier into the system is inevitable, and, to this end, various developments have been made vigorously to permit practical use.
As shown in FIG. 20, the Raman amplifier comprises an optical fiber as an amplifying medium, and a pump source for generating stimulated Raman scattering in the fiber. When silica-based optical fiber is used as the amplifying medium, peak of gain is generated at a longer wavelength side than a wavelength of the pump light, i.e., in a frequency band smaller than that of the pump light by about 13.2 THz. For example, since a difference in wavelength between 1450 nm and 1550 nm corresponds to 13.2 THz, a pump light having a wavelength of about 1450 nm is used in order to amplify the C-band, and a pump light having a wavelength of about 1490 nm is used in order to amplify the L-band (FIG. 21).
However, single wavelength pumped Raman gain has great wavelength dependency, and as apparent from FIG. 22, from when the Raman gain exceeds about 5 dB, it is impossible to suppress gain deviation below 1 dB regarding an operating band width of 30 nm. In order to solve this problem, it is effective that a plurality of pump lights having proper wavelength interval are applied to the Raman amplifier (i.e., the amplifier is pumped by a multi-wavelength pump source). According to this method, Raman amplification having better gain flatness can be achieved in a wider band than the conventional band. As disclosed in Japanese Patent Publication No. 7-99787 (1995) (particularly, in FIG. 4 thereof), such concept itself can be understood intuitively. Japanese Patent Application Nos. 10-208450 (1998) and 11-34833 (1999) refer concrete values of the wavelength interval and assert that the proper value is 6 nm to 35 nm.
FIGS. 23A and 23B show examples of Raman gain profiles obtained when the pump light intervals are selected to 4.5 THz and 5 THz, respectively and DSF is used as an amplifying fiber. As apparent from FIGS. 23A and 23B, when the pump light interval is increased, a valley of gain is deepened and gain deviation is increased. In FIG. 23A, values shown in the following Table 1 were used as frequency (wavelength) of the pump light and, in FIG. 23B, values shown in the following Table 2 were used as frequency (wavelength) of the pump light. In this case, the pump light interval of 4.5 THz corresponds to 33 nm and the pump light interval of 5 THz corresponds to 36.6 nm. That is to say, these examples show the fact that the gain flatness is not so good if the pump light interval becomes more than 35 nm.
TABLE 1Pumping frequencyPumping wavelengthWavelength intervalTHznmnm204.51466.033.0200.01499.0
TABLE 2Pumping frequencyPumping wavelengthWavelength intervalTHznmnm205.01462.436.6200.01499.0
FIG. 24 shows a gain profile obtained when the pump light interval is selected to 4.5 THz and three wavelengths are used. From FIG. 24, it can be seen that, when the third pumping wavelength is added, the valley of gain is deepened in case of the pump light interval of 4.5 THz. FIG. 25 shows a gain profile obtained when the pump light intervals are selected to 2.5 THz and 4.5 THz, respectively and three wavelengths are used. In comparison with FIG. 24, the valley of gain becomes shallower. Since the frequency interval of 2.5 THz used here corresponds to the wavelength interval of about 18 nm, also in this case, the wavelength interval is included in the range from 6 nm to 35 nm. However, considering conversely, it can be said that, even when the wavelength interval is included in the range, if the wavelength interval is not set properly, flat gain cannot be obtained.
By the way, in designing the conventional WDM optical amplifiers, the object was to reduce the gain flatness as small as possible, and an optical amplifier in which all of optical signals are subjected to the same gain was ideal. When the number, power and band width of the used signals are small, such design concept is adequate. However, as the used band of the optical signal is increased, there arose a problem regarding Raman amplifying effect between optical signals. As disclosed in journal (for example, S. Bigo et al, “IEEE Photonics Technology Letters”, pp. 671-673, 1999), in this phenomenon, WDM signals which were set to have same powers upon incident on a transmission line tend to include linear tilt in which power becomes small at a short wavelength side and great as a long wavelength side after transmission. Such tilt is determined by various factors such as the number, power and band width of the optical signals, property of a fiber constituting the transmission line and a transmission distance. As means for coping with this problem, there has been proposed an tilt compensator (T. Naito OAA'99, WC5) for attenuating the long wavelength side signal by using a loss medium having wavelength dependency and a method (M. Takeda et al, OAA'99, ThA3) for compensating tilt by give relatively great gain to the short wavelength side signal by using wavelength dependency of Raman gain. Since the former method for giving the loss has disadvantage due to noise, the later method is more excellent. However, in the paper written by Takeda et al, since the tilt of Raman gain is not linear, the gain flatness after compensating the tilt is relatively great (more than 1 dB).
Similar to the WDM system using only the above-mentioned EDFA, also in the WDM system using the Raman amplifier, when the WDM system for C-band is introduced, it is desirable that the system is designed so that the WDM system for L-band can be added while maintaining the function of the equipment.
In a Raman amplifiers using wavelength multiplexed pumping, when it is desired to expand the gain band, like band extension from C band to C+L band, it is necessary that, while utilizing all of pumping wavelengths which were used before expansion, after the expansion, the amplifier can be operated for the C+L band. That is to say, it must be designed so that, by adding new pumping wavelength to the pumping wavelength used for the C-band, the amplifier can be operated for the C+L band. In this case, it is necessary that wavelength arrangement for flattening the gain in the C-band and the C+L band can be commonly used.
Since the gain deviation is proportional to the magnitude of the peak gain, if the gain is great, the pump light interval must be set small. Further, as mentioned above, in a case where the pump lights are equidistantly arranged, even when the wavelength interval is smaller than 35 nm, the gain deviation may not be reduced sufficiently. Also in this case, it is necessary to use narrower wavelength interval. Although the gain deviation can be reduced by reducing the pump light interval in principle, due to problems regarding a wave combining technique and cost, practically, the pump light interval has a lower limit. In Japanese Patent Application Nos. 10-208450 (1998) and 11-34833 (1999), the lower limit is determined to 6 nm on the basis of the wave combining technique.
However, in the above-mentioned Japanese Patent Application Nos. 10-208450 (1998) and 11-34833 (1999), although the fact that the interval between two adjacent pumping wavelengths is preferably within a range from 6 nm to 35 nm is disclosed, adequate information regarding detailed design values is not disclosed. Further, in the design described in a published paper (Y. Emori et al, OFC'99 PD19), the gain deviation is 1 dB, and this technique cannot be applied when smaller gain flatness is required.